Dependability Optimization of Process-level Protection in an IEC-61850-Based Substation

Power substations are intensively renovated toward using information and communication technologies such as object oriented modeling and Ethernet networks. In the last two decades, Substation automation systems used capabilities of network communication services adopted from sophisticated international standardizations such as IEC 61850. Distributed safety related functions take advantage of these technologies to protect the process-level equipment. Substation devices such as intelligent electronic devices, measurement units and circuit breaker controllers, with new capabilities, i.e. enabling IEC 61850, are integrated to build the protection and control functions that form the safety-related system. The objective of this research is to evaluate quantitatively the dependability for transformer protection architectures in the bay level. Safety integrity levels model, described in both IEC 62061 and IEC 61508, gives measurements for safety integrity levels according to the probability of failure. The determination of these levels is an approach to estimate system dependability. to maintain the health of the power grid. Smart technologies and intelligent devices protect and control substation functionalities either locally or remotely by means of information and communication technologies. The IEC 61850 standard, communication networks and systems in substations, drives interoperability between several substation devices, e.g. intelligent electronic devices (IEDs), from different vendors. Additionally, the standard provides flexible assignment of different functions that enhance safety-related functions (Brand et al 2004). 3 THE IEC 61850 STANDARD The technical committee 57 (TC57), which belongs to The International Electro-technical Commission (IEC), has released the IEC 61850 standard to enforce interoperability and to enable abstraction of communication services (IEC TC57. 2010). Its parts manage standardization of different services dedicated for vertical and horizontal communications inside substation levels. It enables delivery of highspeed peer-to-peer messages for status and events exchange. The standards includes at least 10 parts, the first five parts identify general and specific functional requirements. Part 6 covers an engineering tools such as substation configuration language to manage the design configurations by allowing description of relations between substation functions, substation primary or/and secondary equipment. Part 7 includes 4 sub parts identifying abstraction and mapping of data services to communication protocols. Part 8 defines manufacturing message services (MMS) mapping to data services. Part 9 also has sub parts, part 9-1 defines mapping of sampled values (SV) and 9-2 deals with the Process-bus. Part 10 emphasizes procedures of conformance testing. 3.1 IEC 61850 Standard features The substation automation systems benefit from the standard parts at different levels (station, bay and process levels). Inherited engineering models are enforced by the standard providing new capabilities such as object-oriented data modeling, communication mapping services, configuration tools such as substation configuration language, and new processlevel instrumentation technologies. These capabilities paved the way for horizontal communications between distributed devices inside substation levels. In result, the IEDs cooperate in real-time, to enable distribution of functionalities, by using Ethernet network technology. The substation primary equipment is controlled by dedicated formal device function numbers (IEEE std. C37.2 2008). The IEC 61850 part 5 introduces standardized logical nodes (LN) to define these functions. Logical nodes are embedded into the IEDs to form logical devices that are defined by specific requirements. Obviously, the IEDs are programmable devices that provide protection and control functions. These devices contain logic solvers, input and output ports, network interfaces to gather information about the primary equipment via communication protocols. GOOSE (general object oriented substation events) messages are multi-casted from publishers (IEDs) to subscribers (IEDs) in real-time pattern. These messages are important for delivering status (data set) about primary equipment parameters, e.g. circuit breaker switch position and status. Other specific messages called sampled values (SV) are used to inform baylevel devices about the process-level measured physical parameters, i.e. voltage and current. Innovative merging units (MU) are used to convert analog parameters and to transmit synchronously the sampled values with precise time-synchronization (IEC 61850 part 9-2). 3.2 The IEC 61850 object modeling The standard defines object models to enforce interoperability between IEDs from different vendors. These models help reducing costs and time required for configuration of SAS implementations, and improving engineering documentation. As mentioned earlier logical nodes are models that form basic functionalities. A logical node is the smallest part of a function that exchange data (IEC TC 57. 2010). It contains data object and methods used to create functional components of different classes. These classes provide different protection, measurement, control, monitoring functions in SAS operations. Among these IEC 61850 defined classes are XCBR circuit breaker, XSWI isolator or earth switch, TCTR Current transformer, YLTC power transformer, PTOC time overcurrent protection and ATCC automatic tap changer controller. Interoperability between logical nodes achieved through standardized data objects included in every logical node. Three levels of data modeling and services are considered. The first level is the abstract communication service interface (ACSI) that allocates models and associated services for accessing data and object elements. The second level is common data classes (CDC) that specify data attributes while the third level defines logical node classes and data classes. These levels represent hierarchical levels (Fig.1). Alongside this concept, an intelligent electronic device, i.e. physical device PHD, could be constructed by one or several logical devices. Figure 1. Hierarchical object modeling 3.3 The distribution substation under study We choose the transformer bay (Fig. 2), in distribution substation, as a case study to analyze the functional components according to the IEC 61850 standard. We start by identifying specific components such as primary and secondary equipment. The primary equipment is the main process-level circuits composed of a bus bar, power lines, feeders and transformers, while secondary devices are bay-level auxiliary devices such as monitoring, protection and control devices. The bay-level in the substation is used logically between station-level and processlevel equipment. The station-level consists of engineering computers, human machine interfaces, supervisory control and data acquisition, tele-control access, whereas the process-level is composed of electrical power equipment, e.g. circuit breakers. Planning a SAS design requires drawing of a single line diagram (Fig. 2) that shows process-level components. Electromechanical equipment such as disconnectors and CBs are shown. The power transformer, i.e. converting high voltage into a lower voltage levels, converts 34.5 kV into 13.8 kV. This transformer bay forms a small distribution (D1-2) substation architecture (IEC 61850-1 2010). In other hand, additional components: bus bar, line, breakers and disconnectors are interconnected to construct the primary switchgear. These components are controlled by local/remote commands via a local Ethernet network. The bay-level would include protection and control IEDs that handle functionalities of the process-level and gather information about the equipment. The protection and control IEDs are interconnected via communication network (Fig. 2). 4 DEPENDABILITY OF SAS SYSTEMS Dependability is defined as the trustworthiness of a computer system such that reliance can justifiably be placed on the service it delivers. The service delivered by a system is its behavior as it is perceived by its users; a user is another system (human or physical) which interacts with the former (Laprie 1992). Additionally, it is defined as the ability to perform as and when required (IEC 60050-191). Dependability is a collective set of time-related performance characteristics that coexist with other requirements such as output, efficiency, quality, safety, security and integrity (Hardeveld & Kiang 2012). 4.1 Reliability and availability of process level Reliability is defined as continuity of service, in other definition it is defined as a measure of continuous delivery of correct service or, equivalently, of time to failure (Laprie 1992) while IEC TC56 committee defines reliability as the probability that an item fulfils the required functions for the required duration. In power utility communication systems, IEC 61850 part 3 section 4 considers reliability as quality requirement by focusing on communications for substation automation networking services. The standard provides a reference for other standards such as IEC 60870-4 that specifies performance requirements for tele-control. Further, IEC 61850 identifies the reliability of communications, inside the substation different levels, as data exchange without failure, loss or delay of critical messages. Precisely, there should be no single point of failure (SPOF) in substation networks. If failures exist, outcomes may cause damage to substation equipment. The standard insists that communication reliability is needed as a requirement for substation automation; therefore, it recommends fail-safe design that should be handled to avoid undesired control action. The availability is defined as readiness of usage, and as a measure of the delivery of correct service with respect to the alteration of correct and incorrect service (Laprie, 1992). IEC TC56 illustrates: “the availability is extent to which an item is operational and able to perform any required function or set of functions if a demand is placed on it”. Evidently, one can recognize the relation between the dependability attributes. PHD